By KEAY DAVIDSON, SAN FRANCISCO CHRONICLE

Published 10:00 pm, Monday, August 5, 2002

An unknown infectious agent may be responsible for a fivefold increase in mental illness over the past two centuries, a psychiatrist proposes in a new book.

The claim by Dr. E. Fuller Torrey, a psychiatrist-author and scourge of mainstream psychiatry, challenges common explanations that mental illness is caused by a combination of genetic factors and environmental influences, such as family upbringing.

Torrey's new book also defies a school of academic thought that rates of at least one major type of mental illness, schizophrenia, have sharply declined or remained stable.

To test his hypothesis, Torrey and his associates are quietly investigating whether they can ease symptoms of mental illness with anti-viral and anti-parasitic drugs. More than 100 volunteers are involved in the trials at SheppardPratt Health System, a psychiatric hospital in Baltimore, and at a psychiatric hospital in Addis Ababa, Ethiopia, where Torrey was a Peace Corps physician in the 1960s.

Torrey, 65, is also a psychiatry professor at the Uniformed Services University in Bethesda, Md. From 1970 to 1975, he was a special assistant to the director of the National Institutes of Mental Health. Over the years, he has antagonized figures on both sides of debates over mental health care and involuntary commitment of the mentally ill.

His new book, "The Invisible Plague: The Rise of Mental Illness from 1750 to the Present" (Rutgers University Press), co-written with his research assistant Judy Miller, has received respectful reviews.

Traditionally, debates over the origin of mental illness have tended to fall into two broad camps, "nature" and "nurture." The nature approach holds that biological factors, such as genes and biochemical influences, control mental illness. The nurture approach emphasizes environmental factors such as family upbringing.

The most famous nurture explanation, one that dominated U.S. psychiatry after World War II, came from psychoanalysis, especially the Freudian school, which emphasized the importance of childhood sexual experiences. "There was no scientific evidence to support it," Torrey and Miller assert.

Today, nature theories attract more attention, especially with the purported success of psychiatric medications such as Prozac, and the studies that claim to link certain genes to specific mental illnesses.

Torrey acknowledges that mental illness, like most illnesses, is influenced by genetic tendencies. Yet genes are not overriding: Even identical twins can markedly differ in their degree of mental health.

Furthermore, Torrey and Miller write, that from the mid-19th century to the mid-20th century "most individuals with schizophrenia and manic-depressive illness were confined to asylums for the majority of their reproductive years. Their rate of procreation was extraordinarily low, and so the transmission of their genes was infrequent. Yet ... schizophrenia and manic-depressive illness increased rapidly."

And the fact that scientists have had difficulty finding a specific genetic factor -- one that is consistently replicated in studies "is a strong argument against these diseases being primarily genetic in origin."

The best explanation for the trend, they argue, is biological. They suspect an unknown infectious agent, and possibly other biological factors, such as "changes in diet and exposure to toxins." Growing population and urbanization allowed the infectious agent -- a virus or parasite -- to spread more rapidly in the densely populated cities, they say.